The symbol of Voldemort’s daughter Delphi, known to her followers as The Augurey (CC3.16).

Delphi wears a tattoo of the body and wings of an Augurey, a mournful bird that foretells death, on the back of her neck (CC3.16).

Hermione told Harry that Giants with winged tattoos on their backs had been walking through the Greek Seas (CC1.5). At the time, they did not know about the connection of the Augurey symbol to Delphi and Voldemort.